1. Field of the Invention
The present invention relates to an electronic discharge machining (EDM) process for forming a surface layer having a mirror-finish on a workpiece. Particularly, it relates to a process for forming a surface layer by electric discharge machining, which provides a finely-machined surface and improves the surface's resistance to corrosion and wear by promoting discharge dispersion at the surface of a workpiece material and conducting surface treatment of the workpiece during the EDM process.
2. Description of the Background Art
A process is known (see Japanese Patent Disclosure Publication No. 24916 published in 1987) in the art of electric discharge machining which uses an electrode formed of a submetal material (i.e., a semiconductive material), such as silicon. During EDM, the submetal electrode forms a solid surface layer that is not susceptible to aqua regia and is difficult to damage, e.g., it is not spallable or easily cracked when subjected to several tons of force. This known process employs an ordinary electric discharge machining system with a submetal electrode, conducting machining on a workpiece made of SUS304 (18Cr--8Ni stainless steel), 13Cr steel or high-speed steel. A highly corrosion-resistant surface is formed on the surface of the SUS304, 13Cr steel or high-speed steel by carrying out such machining for several minutes to several hours.
Further, adding a mixture of metallic or submetallic (semiconductor) powder into the machining dielectric fluid improves the stability of discharge. Moreover, the degree to which the mixture enhances the mechanical properties (e.g., corrosion resistance and wear resistance) of the electrode and workpiece surfaces depends on the material being mixed in. It is thus possible to employ the electric discharge machining process for the surface treatment of metal, in addition to its conventional use in metal removal. The type of powder material used is, for example, a semiconductor material such as silicon.
In general a high-voltage superposition circuit is employed as the machining power supply. When the voltage of the high-voltage superposition circuit is large, cracking and/or pitting occur less in the workpiece surface. Further, if silicon powder is present in the machining gap, an electrical spark is generated more easily over a longer machining gap distance, even if the applied voltage remains constant. However, applying a higher voltage will further stabilize machining. The corrosion and wear resistance of a workpiece machined in such a manner improves considerably.
The superposition of a voltage of approximately 100 to 400 V has been shown to stabilize machining and suppress cracking and pitting. This leads to a considerable improvement in corrosion resistance and wear resistance. Moreover, surface roughness is also reduced. However, the powder material breaks down during discharge operations and will usually reach its life expectancy after about 100 to 200 hours of use.
In addition, while the mixture of the powder suppresses cracking and pitting, enhances corrosion and wear resistance, and reduces surface roughness, these effects are not consistently reproduceable under any given machining condition. Specifically, experiments have shown that the mixture of the powder enhances the above noted effects by a greater amount when the applied voltage is low. The effects decrease abruptly when the voltage moves beyond a certain applied voltage. More specifically, the surface roughness increases greatly as the applied voltage increases.
An electrode employed in a conventional method (FIG. 13) comprises a silicon plate bonded to a copper rod 2 with an electrically conductive bonding agent 3. The electrode may be formed by mechanically machining a silicon block or by discharge-machining a silicon block.
When a metal mold is processed, according to the above-described method, a copper or graphite electrode that has a low consumption rate is used to roughly shape the surface and then a silicon electrode is used to finish the surface. This two step process ensures that the mold's surface is corrosion-resistant and wear-resistant. Two steps are required since silicon material is expensive, and a silicon electrode is quickly consumed during the discharge machining operation (approximately ten times as fast as the copper or graphite electrode). Thus, it is not economical to use a silicon electrode in EDM operations which remove a large amount of material from the workpiece.
In other words, a strong surface cannot be formed on the workpiece without using two different electrodes, such as a shaping electrode (made of copper or graphite) and a finishing electrode (made of silicon).
In view of the above disadvantages, it is desirable to provide a method for forming a workpiece surface, by using the shaping electrode of copper or graphite, which has the same characteristics as a workpiece surface formed by the finishing electrode of silicon.